Borane is one of the most useful and widely used organic reagents. Borane enables the hydroboration of alkenes and alkynes, and a myriad of chemical transformations arising from the so-formed organoborane derivatives. Borane also enables reductions of a host of other organic functionalities. The instability and low reactivity of diborane toward alkenes has resulted in the development of numerous more reactive borane complexes. In addition, the pyrophoric nature of borane renders both its use and transportation hazardous. Therefore, borane typically is coordinated, e.g., to tetrahydrofuran (BH3.THF)1,2 or dimethyl sulfide (BH3.SMe2, i.e., BMS)3-6, with BMS being preferred because of its greater stability.
The hydroboration of alkenes and the reduction of numerous organic functional groups by borane-dimethyl sulfide (BMS) are extremely common transformations in organic synthesis.2-4 This includes the use of BMS as a relatively stable borane carrier for the generation of chiral boranes, such as diisopinocampheylborane, β-isopinocampheyl-9-borabicyclo[3.3.1]nonane, and other such chiral boranes.5 It also includes the use of BMS to generate the active species from chiral oxaborolidine precatalysts for use in asymmetric reductions.29 BMS, therefore, is extremely widely used in academic and industrial laboratories, but much less so in industrial processes.
Although BMS is considerably more practical to use than borane itself, BMS suffers from the disadvantage of liberating stoichiometric amounts of dimethyl sulfide in the course of its reactions. Dimethyl sulfide is an extremely volatile (b.p. 37° C.) and foul-smelling compound. The generation of dimethyl sulfide also is environmentally unacceptable, which makes BMS-mediated reactions undesirable for large commercial applications. For example, if used in a large reaction scale, a BMS mediated reaction would require scrubbing of the waste stream. BMS also has stability problems and is pyrophoric.
These disadvantages enormously restrict the use of BMS on an industrial scale. Attempts to overcome these problems include the use of water-soluble sulfides and nonvolatile sulfides, 8,9 but these approaches are deficient in terms of recyclability of the sulfide. Also, see H. C. Brown et al., J. Org. Chem., 51, pp. 4970-4976 (1992).
For example, the disadvantages of BMS prompted investigators to use 1,4-thioxane7 as a less volatile carrier for borane, and, more recently, bis(hydroxyethyl) sulfide,8 which has the additional advantage of being water soluble, and therefore is conveniently removed by aqueous extraction following reductions using BH3.S(CH2CH2OH)2. Other sulfides that have been used as carriers for borane include 1,2-bis(tert-butylthio)ethane and 1,4-bis(benzylthio)butane.9 
The present invention, therefore, is directed to improved methods of hydroborating alkenes and alkynes and of reducing various organic functionalities using borane in a large scale, and that overcome the disadvantages associated with prior synthetic methods using a borane-sulfide complex. The present invention particularly is directed to using 2-(perfluorooctyl)ethyl methyl sulfide and similar fluorous sulfides as readily prepared, odorless, nonflammable sulfides for complexation and stabilization of borane. The fluorinated sulfides also are readily recycled.